We have generated a transgenic mouse in which over-expression of the short isoform of the tumor suppressor p53 (p44) results in the premature loss of health and life. We intend to delineate the molecular mechanism by which increased p44 expression accelerates the process of normal aging in this animal model. In SPECIFIC AIM 1, we test the hypothesis that a change in the p44-to-p53 ratio can modulate mammalian longevity. In this Aim, we characterize the expression of p44 during normal aging in both mouse and human (Experiment 1.1) and determine its relevance in the context of full-length p53 to the rate of aging using RNAi in cells (Experiments 1.2) and a p44-deficient p53 allele in animals (Experiment 1.3) to manipulate p44 levels. In SPECIFIC AIM 2, we test the hypothesis thatp53 modulates life-span in the mouse by coupling the Sir2 and IGF-1 pathways. In this Aim, we propose a series of biochemical experiments to elucidate the molecular mechanism by which p44 might interfere with the interaction between p53 and Sir2, including altering the interaction of p53 with its known binding partners (Experiment 2.1); altering p53 post- translational modifications, including phosphorylation and acetylation/deacetylation (Experiment 2.2); altering p53 sub-cellular localization (Experiment 2.3); and, interfering with the assembly or function of p53- dependent transcription complexes (Experiment 2.4). If p53 is the link between Sir2a and IGF signaling in the mouse, then p44 uncouples the link, and the clues to how this link functions normally lie in why it is non- functional when p44 is in excess. Finally, in SPECIFIC AIM 3, we test the hypothesis that the Igf-1 receptor is theprincipal downstream effector by which p44 alters mammalian health- and life-span. In this aim, we exploit the power of mouse genetics to determine if the Igf-1 receptor, which is altered in cells and tissues ofp44 homozygous transgenic mice, is the key component mediating the loss of health and life in this mammalian system. Because this gene is the critical first step in IGF-1 signal transduction, this Aim focuses on the interactionof p53 and the signalingpathway that plays a major role in life-span determination in lower organisms, such as C. elegans and D. melanogaster.